These studies are based on an in vitro model of angiogenesis in which blood vessels grow out of embryonic kidney explants. In this system, angiogenesis is markedly stimulated by phorbol esters when the explants are specifically grown in type I collagen. The goals of this study are: 1. To examine the cell matrix interactions crucial for microvessel outgrowth. 2. To assess how phorbol esters stimulate microvessel outgrowth. The long term goals are to gain insight into the process renal vascularization in general, and to the development of the highly specialized glomerular capillary endothelium in particular.